Exercise has the ability to optimize your body's cardiovascular, nervous, muscular, and endocrine systems. Exercise increases the demands on your body and, as a result, your body compensates by increasing muscle tissue, vascular networks, neural connections, and efficiency. For example, aerobic endurance training increases the demand on the heart to pump blood throughout the body delivering oxygen and fuel and removing carbon dioxide and other wastes. Simultaneously, your lungs try to increase its intake of oxygen and its release of carbon dioxide. To meet this increased demand your body will begin to build muscle tissue in the heart and increase the vascular networks throughout out your muscles and lungs. These changes help your body to meet the higher level of demands by allowing the heart to pump harder and the lungs to work more efficiently. You will notice these changes because you will be able to exercise for longer periods of time or at higher intensities before becoming fatigued. Exercise will also enhance your nervous system through the creation of new neural pathways resulting from the novel and increased communication between your central nervous system and muscles. Better coordination often results from these new connections. In addition, exercise creates positive changes in hormone production. Regulation of hormones such as testosterone, insulin, and growth hormone, help maintain the integrity of muscle, bone, and connective tissue.